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How Caffeine Affects the Heart Rate of Daphnia

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The purpose of this experiment is to determine whether or not caffeine has an affect on the heart rate of DaphniaHypothesisThe Daphnia heart rate will increase with the concentration of caffeine.

I based my hypothesis on the fact that humans heart rates increase with caffeine as it acts as a stimulant drug.

In the experiment there will be several variables that I need to try and control, the temperature of the water is one of these because if the temperature is higher or lower on the day the results may end up being higher or lower, this would make my results inaccurate. I need to try and limit the human error in the experiment as the daphnia heartbeat is very quick so it will be hard to be completely accurate so we are using the blind method.

We need to use the same microscope throughout testing as others may have different strength lenses. I am also going to keep the species of daphnia the same, as different species may react differently to the caffeine. I will use the same type of caffeine and record the amount of time the daphnia spend in the caffeine before the counting begins, and try to keep this to a similar range.

The independent variable I will change is the concentration of caffeine in the water. I am going to be testing 0.1%, and 0.5% caffeine and also a distilled water trial. This means I will be able to see if the heart rate is greater with caffeine or not. The variable we are going to measure is the rate of the Daphnias heartbeat and to do this we will use a microscope to count the heartbeats of the Daphnia for 15 seconds. This makes it more accurate as the heartbeat is fast so we can the times the result by 4 to get the actual result for 1 minute.

I am going to be using a pipette to transfer the daphnia from the pond water to the cavity slide; this makes it easier for us to transfer the daphnia, without killing them. It also means we can reduce the amount of pond water on the slide making it more accurate. There is ethical issues regarding the daphnia, this means we will just perform the experiment in the quickest time possible, so as not to do any harm to them. I will just perform the experiment as quickly as possible because too much caffeine for too long could harm the daphnia. Using a pipette also reduces the risk of harming the daphnia as they are so small we cannot pick them up with our hands or we would injure them. We used distilled water aswell on cotton wool to give the daphnia a sufficient supply of oxygen so they can survive.

There are also many risks with this experiment, we need to make sure all bags and stools are under the desk. We also need to be careful of dropping any glass and wash our hands after handling the daphnia as they are live living organisms.

EquipmentCulture of DaphniaCavity slidesDropping pipettesDistilled waterCaffeine solution of different concentrationsCotton woolStop clockMicroscopeBeakersMethod1.Get all the equipment as shown above2.Place a few strands of cotton wool on a cavity slide, this restricts the movement of the daphnia3.Add a couple of drops of distilled water to the slide4.Use the microscope to look at the daphnias heart beat5.Use a stopwatch to record the number of heartbeats per 15 seconds6.It is easiest to tap a pencil on a piece of paper and count up the pencil marks at the end and times this number by 4 to get results for 1 minute7.It is best to use a blind method, where the person who is counting is unaware of which solution has caffeine8.

Repeat the procedure using other daphnia and use several different concentrations of caffeine9.Repeat each concentration 3 times for accuracy10.Get the average heartbeat for each concentrationSafetyWear safety gogglesWear lab coatsWash hands at end of experiment as we have been dealing with living organismsFair TestTo make it a fair test we will use the blind method where the person who is recording the number of heartbeats does not know the concentration of caffeine which they are measuring. We will also take 3 readings for each trial to make it more accurate then find the average for all the groups. Also when measuring the daphnias heart beat we are counting for 15 seconds then multiplying the result by four to make it a result for 1 minute.


My hypothesis was that as the amount of caffeine increases the heart rate of the daphnia would increase. The results indicate that my hypothesis should be accepted as on every group the heart rate increased with a larger concentration even though they were all different. This shows that the method of using the dots tapping was not very accurate making the results unreliable. The reason I think the results are different for the different groups is that everyone is doing different counts as there were different people counting the heart beats for each group.

The group labelled should be used as anomalies as they do not fit in with the trend of the rest of the results, this will make the average more inaccurate. Also this group has not included the results for 0.5 making the data pointless. The group labelled also seems to have 2 inaccurate results at distilled water and 0.5% caffeine so I would also class these as anomalies. Therefore this leaves us with just two sets of results. This means that we need to collect more data for us to be able to get an idea of accurate readings.

EvaluationIf I were to do the experiment again I would find a more accurate way of counting the heartbeats as they ended up being very fast. One way I could do this would be to use videoing microscope to record the daphnia heartbeat then use a computer program to slow the heartbeat down so I could count it accurately.

I could also do more trials to make the readings even more accurate. I would try usingDifferent types of caffeine such as decaf and also try using a different species of daphnia and even see if temperature had an effect. I would also try to do many more different concentrations of caffeine.

As I know from my knowledge caffeine is a stimulant so I could experiment using a depressant such as alcohol and see how this affects the daphnia.

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